High shear viscometry of concentrated solutions of poly (alkylmethacrylate) in a petroleum lubricating oil

Summary

High shear capillary viscometry at 37.8°C (100°F) of concentrated solutions of a series of polyalkylmethacrylate viscosity index improvers in a petroleum lubricating oil is reported. Viscosity average molecular weights of the four polymers varied from 355 000–1650 000 and solution concentrations varied from 2–20 wt.-%. An approximating function based on the error function was computerfit to the complete flow curves by correlating the distribution of apparent viscosity with the product ($\dot \gamma$τ), the rate of viscous energy dissipation. This gave an estimate of the secondNewtonian viscosity (η _∞ ) and two parameters of the approximating function. The fourth quantity required to completely define the flow curve is the low shear or firstNewtonian viscosity (η ₀ ). Representation of the original data was within 2%, by this technique.

The parameters of the flow function — the energy level at the inflection point and the slope of the transformed flow function — were found to vary in a regular manner with both molecular weight of the polymer and polymer concentration, expressed as relative viscosity (η _rel). The limiting asymptotes of the approximating function —η ₀ andη _∞ — could not be treated according to the conventionalHuggins equation, but they were fit adequately by theMartin equation: log(η _sp/C)=log[η]+K[η]C. The intrinsic viscosities thus determined for both low shear ([η _M ]₀) and high shear ([η _M ]₀) demonstrate aMark-Houwink relationship, i.e., [η _M ]₀=5.668×10⁻⁵ M ^0.660 _v and [η _M ]_∞=2.574×10⁻⁵ M ^0.0669 _v so that ([η _M ]_∞≈[η _M ]₀/²) over the range studied. The relationship of these results to other reports of high shear viscometry of polymer solutions is discussed.